[Bug] Date picker: February displays incomplete set of days about attractions

[illright]

Cannot reproduce on desktop/mobile Chrome and on Firefox. Could you tell me what's your browser and system locale?

[sallaben]

Mozilla/5.0, Gecko/20100101 Firefox/85.0 -- for what it's worth, I can reproduce on mobile (Safari). Maybe it's a locale based bug? I'm in USA

[aabounegm]

I also couldn't reproduce on Firefox 84 or 85.0.2, and my locale is also set to en-US. I have no access to Safari to test it there, though.

[sallaben]

Very interesting, thanks for the comments guys... I looked deeper into the calendar component and datetime utils, and the main issue boils down to the setMonth functionality on JS Date.

  const foo = new Date(0);
  foo.setFullYear(2021, 1); // setMonth is used by setFullYear implementation as well
  console.log(foo);

We expect this return a Date object for February 1st, and when I run this code in a UTC node container, I get the expected result: 2021-02-01T00:00:00.000Z. But when I run the exact same code locally in node (15.8.0) or in my browser, the result is 2021-03-04T00:00:00.000Z.

First: I'm thinking maybe the way we create the day cursor, using new Date(0), is failing in non UTC environments (I am in Mountain Standard Time). But this should only account for at maximum 1 day difference.

Here's the other problem, from what I understand. When the date is, for example, January 31 2021, and we setMonth to February, it becomes February 31 2021 -- which then becomes sometime in the first week of March 2021, since there is no February 31. This messes up the rest of getCalendar logic downstream.

• Here are two StackOverflow questions about this topic. 1 2

I have identified a fix for these behaviors. When using setMonth or setFullYear to change the month value, ensure that we also pass a date value in at the same time. Also create day cursor using new Date() and not new Date(0).

• Here is a PR to fix the behavior in the datetime-utils: #244

And February now displays normally :-) ...Sorry, I am a MM.DD.YYYY weirdo.

[aabounegm]

It was confusing to me how these links are relevant when the date today is the 15th (or 14th for you), and especially that new Date(0) should return the epoch (1970.01.01), irrelevant of the current timezone (I was wrong!). Turns out that the timezone actually affects even that constructor, so I tried changing my timezone to US/Pacific (the one available in DevTools) and running new Date(0), and it indeed gave me 1969.12.31!
Thank you for pointing this out, I'll review the PR now.

P.S.: I can get used to MM.DD.YYYY, but I've never seen a locale where the week starts on Friday!